Rub it down with fine grade sandpaper until you get it as clean as you possibly can, preferably with the whole ball shining clear metal, and then clean it off with brake cleaner (preferably) or white spirit. Don't hook up for a day or so afterwards, as there will be a chance that the cleaning fluid residue may not be totally evaporated, and this can lead to horendous creaking noise, which will make you think that something seriously is wrong.I made this mistake recently prior to going away one week-end, and the outward trip was most annoying-the homeward trip two days later was totl silence.
To keep it clean, just use a good tow-ball cover, and wipe it occasionally with a clean cloth.If it starts to corrode over, then it's back to the sandpaper and cleaner I'm afraid.